Why would a Nazi chief bother telling Americans what they would have known already? If the US deliberately used Spanish Flu against German soldiers the CIA would already have known this.

What was he trying to do, insult his interrogators?

The US Army has its own intelligence department, separate from the CIA. Although they usually coordinate their activities and share information, I can see why the US Army wouldn't necessarily want to admit to a blunder of this magnitude - even to a sister intelligence agency...